Power control circuit for resistance heating moving conductors

ABSTRACT

In apparatus in which an elongated electrical conductor is heated by the Joule effect as it is moved between two or more spaced electrical contacts, an improved circuit for regulating the temperature to which such conductor is heated. The power supplied to heat the conductor is controlled in response to a comparison between the square of the current passed through the conductor and the speed of the moving conductor to maintain substantially constant the temperature to which the conductor is heated over a wide range of conductor speeds.

United States Patent [191 Ellinghausen et a1.

[ 51 Oct. 15, 1974 POWER CONTROL CIRCUIT FOR RESISTANCE HEATING MOVING CONDUCTORS [75] Inventors: Edgar A. Ellinghausen; George B. Johnson, both of Crystal Lake, 111.

[73] Assignee: Interstate Drop Forge Co.,

Milwaukee, Wis.

[22] Filed: Aug. 24, 1973 [21] Appl. No.: 391,128

Related US. Application Data [63] Continuation of Ser. No. 313,500, Dec. 8, 1972, abandoned, which is a continuation-in-part of Ser. No. 203,581, Dec. 1, 1971, Pat. No. 3,739,132.

[52] US. Cl. 219/155 [51] Int. Cl C2ld 9/62 [58] Field of Search 219/50, 155, 108, 116,

[56] References Cited UNITED STATES PATENTS 2,669,647 2/1954 Segsworth 219/l0.75 X

FlLTER 3,311,734 3/1967 Peterson 219/50 3,333,178 7/1967 Van Allen et a1. 321/38 3,398,252 8/1968 Bock et a1 2l9/l0.61

3,476,910 11/1969 Leath et al 219/50 X 3,621,392 11/1971 Liebermann et a1 324/127 X 3,740,859 6/1973 Patton et a1. 2l9/l0.77 X 3,746,825 7/1973 Pfaffmann et a1, 219/10.77

Primary Examiner-J. V. Truhe Assistant ExaminerClifford C. Shaw Attorney, Agent, or Firm-Henry K. Leonard 5 7 ABSTRACT In apparatus in which an elongated electrical conductor is heated by the Joule effect as it is moved between two or more spaced electrical contacts, an improved circuit for regulating the temperature to which such conductor is heated. The power supplied to heat the conductor is controlled in response to a comparison between the square of the current passed through the conductor and the speed of the moving conductor to maintain substantially constant the temperature to which the conductor is heated over a wide range of conductor speeds.

8 Claims, 2 Drawing Figures PAIENIED 3.842.239

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AMPLlFlER FIE-Z- 1. POWER CONTROL CIRCUIT FOR RESISTANCE HEATING MOVING CONDUCTORS CROSS-REFERENCE TO RELATED APPLICATION This is a continuation of our copending application Ser. No. 313,500 filed Dec. 8, 1972, now abandoned which, in turn, is a continuation-in-part of our copending application Ser. No. 203,581, filed Dec. 1, 1971, now US. Pat. No. 3,739,132.

BACKGROUND OF THE INVENTION This invention relates to electrical heating and, more particularly, to an improved power control circuit for controlling the temperature to which a moving conductor such as an elongated wire or rod is electrically heated by the Joule effect.

A variety of electrical resistance heating arrangements have been constructed in the past for resistance heating by the Joule effect moving conductors such as metallic rods, wires, tubes, sheets and the like. Typically, an electric current is passed through the conductor as it moves between two or more spaced electrical contacts for heating the portion of the conductor between the contacts. In a typical prior art wire annealer, for example, a wire to be annealed is brought up to a desired process speed and then a fixed voltage is applied across a region of the moving wire defined by the spaced contacts for heating the wire as it passes through such region. Annealers of this type are inefficient in that a considerable amount of wire is wasted while the wire is initially brought up to speed. In addition, there is no regulation over the wire temperature. Varying conditions such as changes in contact resistance may result in the wire either being heated excessively or being heated insufficiently for annealing.

ln more advanced resistance heaters which include at least a simple regulator, problems have still occurred in regulating the temperature to which the moving conductor is heated. Temperature regulating problems are particularly severe where fluctuations occur in the speed of the moving conductor. If a conductor is collected on a driven spool, the speed of the conductor will increase when the diameter of the spool increases as layers of the conductor are wound on the spool, unless the spool speed is modified to compensate for the increase in diameter. As the speed of the conductor increases, the electric current required to heat the conductor to any given temperature increases. Speed fluctuations may also occur in continuous production lines in which a moving wire, for example, is heated either to relieve stresses or to facilitate drawing the wire through a die.

ln the past, attempts have been made to sense the temperature of the moving conductor and to use the sensed temperature for controlling an electric current which heats the conductor. Such a system is disclosed in Peterson US. Pat. No. 3,31 L734. However, controlling the current applied to the conductor in response to the sensed temperature has not been altogether satisfactory for accurately regulating the temperature to which the conductor is heated. This has been due, at least in part, to the fact that the sensed temperature is affected by the emissivity of the surfaces of the conductor. The emissivity of the conductor is in turn influenced by variables such as the finish and smoothness of the surface and by grease, dirt and other irregular coatings on the surface. Fluctuations in the sensed temperature will cause the system to hunt, resulting in wide fluctuations in the temperature to which the conductor is heated.

Another problem with controlling current in response to the sensed temperature occurs because the temperature of the moving conductor varies as a function of the sensing location between the spaced electrical contacts and as a function of the past current history of the conductor prior to passing the temperature sensor. The portion of the conductor entering the region between the contacts is at its coolest temperature, while the portion of the conductor just leaving this region has been heated to a maximum temperature. Since the important temperature, or control temperature, is usually the maximum temperature, the temperature sensing is necessarily done in the heating region adjacent the final electrical contact. At this point, all of the heating has been accomplished.

When a correction in temperature is called for, the heating rate is changed throughout the entire region between the contacts. It is not, however, until one complete contact spacing of the conductor has passed through the heating region that the sensor gets the full effect of the change that has been produced. At this point, however, there is already another length of conductor between the contacts which has also been partially heated at the new rate and, therefore, an inevitable period of overshoot exists. Due to the total lack of thermal inertia in this method of heating and to the resultant rapid response to changes in the applied heating power, the above phenomena always results in progressively larger oscillations or hunting in the maximum temperature to which the conductor is heated. This phenomena may be reduced by using a slowly responding controller, although this results in other control problems.

Temperature has also been regulated in proportion to the speed of the moving conductor to reduce the effects of fluctuations in the conductor speed and to eliminate possible errors in temperature sensing. Systems of this type have generally been non-linear and have not maintained a constant temperature over a wide range of conductor speeds. Furthermore, the conductor temperature will be affected by changes in properties of the conductor, such as in the cross-sectional area of the conductor which affects the resistance of the conductor and in surface conditions on the conductor which may effect the contact resistance. If only the voltage applied to the conductor is regulated, any changes in contact resistance will have a pronounced effect on the wire temperature since the current through the conductor will depend upon the total resistance of the circuit.

SUMMARY OF THE INVENTION According to the present invention, an improved circuit is disclosed for controlling electrical power used for resistance heating a moving conductor by the Joule effect. It has been found that the temperature of the moving conductor may be maintained substantially constant regardless of the conductor speed by controlling the electrical power supplied to the conductor in response to a comparison between the square of the electric current through the conductor and the conductor speed. By using such a comparison for controlling the applied current, the electrical power which heats the conductor is maintained directly proportional to the conductor speed and hence proportional to the total mass being heated. The conductor temperature is held substantially constant since power is proportional to the square of the current or to the square of the voltage in-an alternating current system. By controlling the applied power in response to current rather than voltage, the adverse effects of variations in contact resistance are minimized. Suitable adjustment means are also provided for compensating the applied power when the conductor speed is at or near zero. Such adjustment means may consist of a manual control or it may be automatic in response to the temperature of the heated conductor in the region between the contacts.

It is therefore a principal object of the invention to provide an improved power control circuit for use with apparatus for resistance heating a moving elongated electrical conductor.

Another object of the invention is to provide an improved power control circuit for use with apparatus for resitance heating a moving conductor in which variations in the temperature to which the conductor is heated are minimized.

Other objects and advantages of the invention will become apparent from the following detailed description, with reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic block diagram of an improved power control circuit for resistance heating a moving conductor constructed in accordance with the present invention; and

FIG. 2 is a fragmentary schematic block diagram showing a modification to the power control circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, apparatus is shown for passing a controlled current through a segment or region of a moving conductor 11 for resistance heating the conductor 11 by means of the Joule effect. The conductor 11 may, for example, comprise a solid wire of an electrically conductive ferrous or non-ferrous material which is heated prior to drawing or otherwise working or a solid or stranded wire which is heated to relieve internal stresses. However, it will be appreciated that the apparatus 10 is not restricted to heating wires and that other electrically conducting articles may also be heated. The conductor 11 passes sequentially between pairs of contacts 12 and 13 which are spaced to form a heating region or zone 14 along the conductor 11. Beyond the contact 13, the heated conductor 11 may be drawn, shaped or otherwise worked or it may be collected upon a Spool.

The contacts 12 and 13 may be of any suitable design for supplying an electric current to the moving conductor 11. However, for heating a wire conductor 11, the contacts 12 and 13 preferably each consists of a pair of disks or rollers 15. The pair of rollers 15 for each of the contacts 12 and 13 are mounted to rotate with the wire pressed between the periphery of the rollers 15. Grooves may be provided in the periphery of the rollers 15 for retaining the wire between the rollers 15 of each of the contacts 12 and 13. The two rollers 15 in each contact pair 12 and 13 are biased together hydraulically or by means of suitable springs (not shown) to tightly pinch the conductor 11 in the grooves, forming a good electrical connection between the conductor 11 and the contacts 12 and 13. One or more optional motors (not shown) may be used for driving the rollers 15 at a constant speed, or the rollers 15 can be rotated by the conductor 11 as it is pulled through the region 14.

Electrical power is applied between the contacts 12 and 13 from a secondary winding 16 of a voltage stepdown power transformer 17. The power applied between the contacts 12 and 13 causes the cold wire entering the heating region 14 at the contact 12 to be heated as it passes through the region 14. The heated conductor 11 then passes between the pair of rollers 15 of the contact 13. The power transformer 17 is energized from a commercial alternating current power source 18 which, for example, may be either 230 volts or 430 volts at 50 or Hz. Power from one side 19 of the source 18 passes through a fuse 20 and a filter 21 to a primary winding 22 of the power transformer 17. Power from a second side 23 of the source 18 passes through a fuse 24, the filter 21 and a power control device 25 to the primary winding 22 of the power transformer 17.

The actual amount of power delivered from the source 18 to the power transformer 17 is determined by the state of conduction of the power control device 25. The power control device 25 may be one or more thyratrons or, preferably, thyristors such as silicon controlled rectifiers. As shown in FIG. 1, the power control device 25 consists of a pair of silicon controlled rectifiers 26 and 27. The controlled rectifiers 26 and 27 are arranged back-to-back, having input and output electrodes connected in parallel between the power source 18 and the primary winding 22 and in reverse polarities. Since the polarities of the controlled rectifiers 26 and 27 are reverse, the controlled rectifier 26 controls power during half cycles of one polarity and the controlled rectifier 27 controls power during half cycles of the opposite polarity. If the appropriate one of the controlled rectifiers 26 and 27 is triggered or fired at the beginning of each half cycle, full power will be applied from the source 18 to the primary winding 22 for passing a maximum current through the conductor 11. If, however, the phase of the firing of the controlled rectifiers 26 and 27 is delayed in each half cycle, the average power supplied to the primary winding 22 is reduced to reduce the average or effective current through the conductor 11. This power control method is sometimes referred to as conduction angle control of the silicon controlled rectifiers.

The power applied to the primary winding 22 of the transformer. 17 must be accurately controlled for maintaining substantially constant the temperature to which the moving conductor 11 is heated. The required power for heating the conductor 11 is a function of the mass of the wire heated at any given time and is primarily affected by the speed at which the conductor 11 is moving. It will be apparent that, if the speed of the moving conductor 11 being heated to a predetermined temperature is doubled, twice the mass will pass through the region 14 in a given period of time and twice the power will be required for heating the conductor 11 to the predetermined temperature. The

speed of the moving conductor 11 is sensed by means of a conventional tachometer 28 which is driven with one of the contact rollers 15. The tachometer 28 generates a signal proportional to the speed of the moving conductor 11 for use in controlling the power applied to the conductor 11.

The output of the tachometer 28 is applied through a span control adjustment and amplifier 29 and a compensation adjustment circuit 30 to a proportioning amplifier 31. The span control adjustment and amplifier 29 consists of a conventional operational amplifier with controls for modifying the input signal from the tachometer 28 and the gain to control the magnitude and the range of the output.

A signal proportional to the square of the current to the transformer 17 is also applied to the proportioning amplifier 31. A current transformer 32 generates a signal which is proportional to both the primary current in the transformer 17 and the current passed through the conductor 11. it will be appreciated that the current transformer 32 may be placed at any convenient location in the primary or the secondary circuit of the power transformer 17. The output of the current transformer 32 is displayed upon an ammeter 33 and is applied to an amplifier 34. The ammeter 33 may be calibrated to indicate the current through the conductor 11. The gain of the amplifier 34 is determined by the actual location or output magnitude of the current transformer 32 to provide a desired signal proportional to the current in the conductor 11. The output of the amplifier 34 is applied to the input of an analog amplifier 35. The analog multiplier 35 is a commercially available integrated circuit which generates an output equal to the square of the input. The output of the analog amplifier 35 is applied to the proportioning amplifier 31 for comparison with the speed signal.

The proportional amplifier 31 is an operational amplifier which has an output proportional to the difference between the square of the current supplied to the transformer 17 and the adjusted output of the tachometer 28. This output is used for triggering the controlled rectifiers 26 and 27 at the proper time or phase in each half cycle to control power delivered through the transformer 17 to the moving conductor 11.

As previously mentioned, the output of the tachometer 28 is modified by the span control adjustment and amplifier 29 and by the compensation adjustment circuit 30. The span control adjustment and amplifier 29 is calibrated to provide a suitable signal to the proportioning amplifier 31 over the speed range through which the conductor 11 may be moved. Compensation adjustment circuit 30 is used for controlling current in the conductor 11 as the speed of the conductor 11 approaches zero. This is due to the fact that a minimum current will be required for heating the conductor 11 to a predetermined temperature when the conductor is not moving. As the speed of the conductor 11 increases from zero, the current required to heat the conductor 11 will be equal to the minimum current required to heat the conductor 11 when stopped plus a current which is functionally related to the speed of the conductor 11.

Since the square of the current applied to the conductor 11 is continuously compared with the speed of the conductor 11, it will be appreciated that the power supplied to the contacts 12 and 13 for heating the conductor 11 will be proportional to the mass of material actually heated in the region 14. Furthermore, since the current supplied to the conductor 11 through the contacts 12 and 13 is monitored rather than the voltage across the contacts 12 and 13, any adverse effects of contact resistance changes are eliminated.

Turning now to FIG. 2, a modification is shown to the compensation adjustment circuit 30 of FIG. 1. In the embodiment of FIG. 2, an optical infrared sensor 36 is positioned to monitor the maximum temperature of the heated conductor 11 prior to exiting the heating region 14 at the contacts 13. The output of the sensor 36 is proportional to the maximum temperature of the heated conductor 11 and is applied through a span control adjustment and amplifier 37 to a summing amplifier 38. The output of the summing amplifier 38 is applied to the proportioning amplifier 31, in a manner similar to that in which the output of the compensation adjustment 30 is applied to the proportioning amplifier 31 in FIG. 1. The summing amplifier 38 combines the adjusted and amplified output of the tachometer 28 and the adjusted and amplified output of the optical sensor 36 for comparison by the proportioning amplifier 31 with the square of the current through the conductor 11. Only a small portion of the power delivered to the transformer 17 is controlled in response to the temperature of the conductor 11 sensed by the optical sensor 36. When the conductor 11 is standing still, the power required to heat the conductor 11 is only a small percentage of the power required for heating the conductor 11 when it is moving at a maximum speed. Only this small percentage of the maximum power is controlled in response to the temperature sensed by the infrared sensor 36. The remaining portion of the maximum power is controlled in response to the sensed speed. Thus, the infrared sensor 36 will only have a major effect on controlling current to the conductor 11 as the speed of the conductor 11 approaches zero. At higher conductor speeds, the signal generated by the infrared sensor 36 will act as a fine tuning" adjustment on the temperature to which the conductor 11 is heated.

If the apparatus 10 is to be used for heating a continuous conductor 11, as in a production line, the speed signal from the span control adjustment and amplifier 29 can be applied directly to the proportioning amplifier 31. Since the conductor 11 will not normally be stopped, there will be no significant waste problem. In this case, there will be an error in the temperature to which the conductor 11 is heated only as the speed of the conductor 11 approaches zero.

It will be appreciated that various modifications and changes may be made in the above-described power control circuit for resistance heating moving conductors without departing from the spirit and the scope of the claimed invention.

What we claim is:

1. In apparatus for resistance heating an elongated electrical conductor which is continuously moved between spaced electrical contacts, an improved circuit for controlling electrical power applied to the conductor through the contacts comprising, in combination, means for applying alternating current power through the contacts to the conductor for heating the conductor by the Joule effect, means generating a signal proportional to the speed of the conductor, means generating a signal proportional to the square of the current applied to the conductor through the spaced contacts,

and means for controlling the power applied to the conductor in response to a comparison of said speed signal and said current square signal.

2. A circuit, as set forth in claim 1, wherein said means for applying alternating current power to the conductor includes a step-down transformer having a primary winding and a secondary winding, meansconnecting said secondary winding to the spaced contacts for supplying power to heat the portion of the conductor located between the contacts, and means for applying alternating current power to said primary winding.

3. A circuit, as set forth in claim 2, wherein said means generating a signal proportional to the square of the current includes a current transformer, means connecting said current transformer to measure the current supplied from said power applying means to said primary winding, said current transformer having an output proportional to the current applied to said primary winding, and means responsive to the output of said current transformer for generating a current signal proportional to the square of the current applied to the conductor.

4. A circuit, as set forth in claim 2, wherein said power controlling means includes amplifier means for comparing said speed signal and said current square signal and generating an output proportional to the difference between said two signals.

5. A circuit, as set forth in claim 4, wherein said power controlling means further includes controlled conduction means, means connecting said controlled conduction means in series with said primary winding and said power applying means, and means responsive to said amplifier means for establishing conduction by said controlled conduction means during at least a portion of each half cycle of the applied alternating current power.

6. A circuit, as set forth in claim 5, including means for generating a signal proportional to the maximum temperature of the portion of 6. A circuit, as set forth in claim 5, including means for generating a signal proportional to the maximum temperature of the portion of the conductor between the spaced contacts, means for generating a signal combining said speed signal and said temperature signal, and wherein said amplifier means generates an output proportional to the difference between said combined signal and said current square signal.

7. A circuit, as set forth in claim 5, including means for combining a predetermined fixed signal with said speed signal, and wherein said amplifier means generates an output proportional to the difference between said combined signal and said current square signal, said fixed signal affecting the temperature to which the conductor is heated as the speed of the conductor approaches zero.

8. A method for maintaining substantially constant the temperature to which a conductor is heated by the Joule effect as the conductor continuously moves through a region between spaced electrical contacts comprising: applying electrical power through the contacts to the conductor for heating the conductor by the Joule effect; generating a signal proportional to the square of the electric current in the conductor; generating a signal proportional to the speed of the conductor; comparing said current square signal with said speed signal; and controlling the applied power in response to the comparison of said current square signal and said speed signal for maintaining substantially constant the temperature to which the conductor is heated in the region.

v r UNITED STATES PATENT- OFFICE CERTIFICATE OF CORRECTION Patent No. 3 842 239 Dated October 15 1974 Inventor(s) Edgar A. Ellinghausen, et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col 5, lines 31 and 32 "ampli-fier" should be multi lier.

Col 5, line 35, "amplifier" should be multiplier.

Col 8, lines 2, 3, 4 delete Signed and sealed this 17th day of December 1974.

(SEAL) Attest:

iZcCOY H. GIBSON JR. C. MARSHALL DAN? Attesting Officer Commissioner of Patents ORM PC4050 HO'SB) I uscoMM-Dc 60376-P69 I I U 5. GOVERNMENT PRINTING OFFICE: I969 0-366-334,

.- I I UNITED STATES PATENT. OFFICE CERTIFICATE OF CORRECTION Patent; No. 3,842,239 v Dated October 15, 1974 Iuventofls) Edgar A. Ellinghausen, et al It is certified that error appears in the above-identified patent and that said Letters, Patent are hereby corrected as shown below:

Col 5, lines 31 arid 32 "ampli-fier" should be multi lier.

Col 5, line 35, "amplifier" should be multiplier.

Col 8, lines 2, 3 4 delete Signed and sealed this 17th day of December I 1974.

(SEAL) ttest' MCCOY If. GIBSON JP. C I IARSHALL DAN?! Attestlng Officer Commissioner of Patents FORM PC4050 I I uscoMM-oc 60376-P69 9 1.5, GOVERNMENT PRINTING OFFICE: 1989 0-366-334, 

1. In apparatus for resistance heating an elongated electrical conductor which is continuously moved between spaced electrical contacts, an improved circuit for controlling electrical power applied to the conductor through the contacts comprising, in combination, means for applying alternating current power through the contacts to the conductor for heating the conductor by the Joule effect, means generating a signal proportional to the speed of the conductor, means generating a signal proportional to the square of the current applied to the conductor through the spaced contacts, and means for controlling the power applied to the conductor in response to a comparison of said speed signal and said current square signal.
 2. A circuit, as set forth in claim 1, wherein said means for applying alternating current power to the conductor includes a step-down transformer having a primary winding and a secondary winding, means connecting said secondary winding to the spaced contacts for supplying power to heat the portion of the conductor located between the contacts, and means for applying alternating current power to said primary winding.
 3. A circuit, as set forth in claim 2, wherein said means generating a signal proportional to the square of the current includes a current transformer, means connecting said current transformer to measure the current supplied from said power applying means to said primary winding, said current transformer having an output proportional to the current applied to said primary winding, and means responsive to the output of said current transformer for generating a current signal proportional to the square of the current applied to the conductor.
 4. A circuit, as set forth in claim 2, wherein said power controlling means includes amplifier means for comparing said speed signal and said current square signal and generating an output proportional to the difference between said two signals.
 5. A circuit, as set forth in claim 4, wherein said power controlling means further includes controlled conduction means, means connecting said controlled conduction means in series with said primary winding and said power applying means, and means responsive to said amplifier means for establishing conduction by said controlled conduction means during at least a portion of each half cycle of the applied alternating current power.
 6. A circuit, as set forth in claim 5, including means for generating a signal proportional to the maximum temperature of the portion of the conductor between the spaced contacts, means for generating a signal combining said speed signal and said temperature signal, and wherein said amplifier means generates an output proportional to the difference between said combined signal and said current square signal.
 7. A circuit, as set forth in claim 5, including means for combining a predetermined fixed signal with said speed signal, and wherein said amplifIer means generates an output proportional to the difference between said combined signal and said current square signal, said fixed signal affecting the temperature to which the conductor is heated as the speed of the conductor approaches zero.
 8. A method for maintaining substantially constant the temperature to which a conductor is heated by the Joule effect as the conductor continuously moves through a region between spaced electrical contacts comprising: applying electrical power through the contacts to the conductor for heating the conductor by the Joule effect; generating a signal proportional to the square of the electric current in the conductor; generating a signal proportional to the speed of the conductor; comparing said current square signal with said speed signal; and controlling the applied power in response to the comparison of said current square signal and said speed signal for maintaining substantially constant the temperature to which the conductor is heated in the region. 